Ultrasonic resonance spectroscopy of composite rings for flywheel rotors

Abstract

Flywheel energy storage devices comprised of multilayered composite rotor systems are being studied extensively for utilization in the international space station (ISS). These composite material systems were investigated with a recently developed ultrasonic resonance spectroscopy (URS) technique. The system, UltraSpecTM, employs a swept frequency approach and performs a fast Fourier transform (FFT) on the frequency spectrum of the response signal. In addition, the system allows for equalization of the frequency spectrum, providing all frequencies with equal amounts of energy to excite higher order resonant harmonics. Interpretation of the second FFT, along with equalization of the frequency spectrum, offers greater assurance in acquiring and analyzing the fundamental frequency, or spectrum resonance spacing. The range of frequencies swept in a pitch-catch mode was varied up to 8 MHz depending on the material and geometry of the component. Single and multilayered material samples, with and without known defects, were evaluated to determine how the constituents of a composite material system affect the resonant frequency.Amplitude and frequency changes in the spectrum and spectrum resonance spacing domains were examined from ultrasonic responses of a flat composite coupon, thin composite rings, and thick composite rings. Also, the ultrasonic spectroscopy responses from areas with an intentional delamination and a foreign material insert, similar to defects that may occur during manufacturing malfunctions, were compared to those from defect free areas in thin composite rings. A thick composite ring with varying thickness was tested to investigate the full thickness resonant frequency and any possible bulk interfacial bond issues. Finally, the effect on the frequency response of naturally occurring single and clustered voids in a composite ring was established.